Cobalamin (vitamin B 12) is an essential micronutrient for the maintenance of normal metabolism. Humans and other animals have evolved a highly specific mechanism for concentrating dietary cobalamin on the absorptive surface of distal small intestinal enterocytes. Cobalamin endocytosis is mediated by cubilin, a large multiligand receptor of the apical membrane. Imerslund-Grasbeck syndrome (I-GS) is an autosomal recessive trait of selective cobalamin malabsorption and proteinuria due to failure to express functional cubilin in the apical membrane of ileal enterocytes and renal proximal tubule cells. Cubilin mutations cause human I-GS in one population, but other causes remain to be determined. An inherited defect of cubilin trafficking to the apical membrane has been described in a canine I-GS model, but cubilin was excluded as the disease gene, indicating that an unknown accessory activity is essential for polarized cubilin expression in intestinal and renal epithelial cells. Our goal is to take advantage of this unique, naturally-occurring animal model to better understand the cell biology of receptor expression in polarized epithelia. Lacking functional gene candidates, we initiated a comparative positional-candidate gene approach to determine the I-GS disease gene. Significant linkage to a gene marker was found, defining an approximately 9 cM region harboring the disease locus. The specific aims of this proposal are: 1) to minimize the region of linkage by developing and applying new genetic markers to a large outbred linkage pedigree in order to identify a subset of genes in the homologous region of the human genome as positional candidates; 2) to clone and analyze positional candidates for mutations; and 3) to initiate functional analyses of the disease-causing gene product. Genes will be chosen for new marker development in an interative process from the locality of markers showing close linkage to the disease locus until a gene marker is found which exhibits no recombination with the disease locus. The I-GS gene will be identified by mutation analysis, and antibody will be generated to study intracellular localization and determine binding partners. Results of these efforts are expected to open a window onto a new aspect of nutrient absorption and membrane receptor biology.